It is well known and recognized by all that the major difficulty which people encounter when they wish to sleep while seated, even when they recline the backs of their seats, is that of being able to place their heads in a comfortable position which is sufficiently stable to prevent the involuntary movements of the head which occur, in various directions, during the unconsciousness of sleep, and which affect the spinal column as well as the muscle mass of the neck, and make it impossible to enjoy relaxed and prolonged rest.
It is also known that the head is itself heavy, and that when people are seated it is held in alignment with the central axis by the musculature of the neck, and is also carried by the spinal column. However, given that the centre of equilibrium is located in the cerebellum, when people are sleeping in this position, and they enter a state of unconsciousness, the cerebellum ceases sending out balancing stimuli to the musculature, while the permanently acting force of gravity, encountering no muscular resistance, presses on the head, turning it in every direction, producing traction forces on the neck and the spinal column, which alternately interrupt sleep, and mean that the persons frequently move their bodies in the search for a support position which neutralizes these effects, and instinctively move their bottoms forward and curve their backs, attempting to rest their heads as far back as possible, raising the chin.
This natural instinct results from the fact that the most comfortable way of sleeping while seated is when the nape passes behind the line of support of the back on the seatback, and the chin is raised to approximately the height or level of the ear lobe, since in this way the weight of the head itself is off-loaded onto the support of the back or seatback, and the spinal column is relieved of the direct vertical load and of the traction forces which are generated when the said weight is located on or in front of the said support line, respectively.
Simultaneously, when the head is placed in the rest position described, the musculature of the neck is relieved, since no frontal restraining forces are required, the persons achieving a comfortable position which, however, is only temporary, since, having their backs curved due to failure to take account of this natural tendency through adequate design of the seat, the whole of the weight falls on the hips, and afflicts people with pain in this region of the body.
The natural and logical instinct to push the nape behind the line of support of the back and to raise the chin to the level of the ear lobes is limited in existing seats by the seatback's own support line, even when the seatbacks have protuberances forward of this line, given that every pattern or element which projects from the support line of the seatback obliges the users to move the head in the opposite direction to that which corresponds to relieving the spinal column of the direct weight of the head and causes a traction force by lever action on the vertebrae and on the muscles of the neck, proportional to the movement made. This means that persons intending to avoid this inconvenience in the design, and seeking an ideal position, move their bottoms forward and automatically reduce the length of useful support, and require more space in front in which to place their legs.
However, it is known that vehicles come equipped with headrests on the backs of their seats, and that they fulfill a function of protecting the passenger, since they prevent the known pendulum effect by the head in the event of an impact. Many of the known headrests can tip up between two positions, one of them being a rest position in which the head is always supported overlying the line of support of the back.
Obviously, all the known headrests comply with the safety standards, but none of them allows the user to rest comfortably while his head is resting thereon, due to the fact that none of them is equipped with a support which is suitable to act as a new point of support and of load distribution which in turn contains the lateral movements of the head during sleep, and makes it possible to push the nape behind the line of support of the back and to raise the chin to the approximate height of the ear lobe, the ideal position for sleeping while seated, and which in turn makes it possible within the same overall measurements of a seat, to achieve an increase in the total useful support surface area, by which, automatically, the necessary space in which to place the legs is reduced, since the forward movements of the body, in the search for comfort, are not necessary.
Argentinian Patent No. 210.394 is known, which attempts to alleviate the above-mentioned drawbacks by means of the provision of a headrest applicable to any type of seat and which has lateral protuberances for partially limiting the movements of the head. It is clearly obvious on reviewing the cited patent, that it does not result in a really comfortable rest position for the user.
European Patent Application 0 133.645 is also known, which discloses a headrest which has a maximum tip-up position in which the user can rest his or her head, with the intention of offering a comfortable position for relaxation and sleep. It is clear on reviewing the cited patent that, in the invention, neither the causes nor the effects which generate and cause the discomforts which people suffer when they wish to relax while seated have been considered. Given that it deals with a headrest which is designed counter to what is required to achieve these conditions, since the central body intended to play a support role lacks a fastening element which actually acts as a support and divides the loads in such a way that the spinal column is not compressed by the heavy intrinsic weight of the head, and which, in turn, prevents the lateral movements of the head, negating any type of traction on the vertebrae and the musculature of the neck. It being obvious that only this body can be employed as a support for the nape, and that, given the erroneous concepts employed in its creation, tipping it to its rest position, forward of the line of support of the back, moving the user's head forward, which means that the whole weight of the head is supported by the spinal column and not by the central body of the headrest, that it does not act as a support, but that it serves only as a rest and, given its faulty shape, neither does it avoid the passenger's moving his legs forward, seeking a comfortable position in which to rest.
It also emerges clearly on reviewing the cited patent, that said headrest is constructed with a fixed perimetral structure. Being linked to the seatback by two rods, and with both the structure and the rods acting as limitations on the movements of the central body, whereby, although it might be wished to at least adopt a position which would make it possible to move the said body rearwards of the line of support of the back, this movement would be made impossible by the design itself, these conditions making it perfectly obvious that it is a headrest which is unsuited and unsuitable for achieving at least the minimum necessary conditions for relaxation, and that it was designed in a form opposed to the objectives sought.
French Patent 2 516.771 is also known, which discloses a design of a cushion overlaid on the back which is fixed thereto by means of a clip, and is formed by a single piece of spongy or inflatable material, and which has a cavity for inserting the head and a curved entry for the neck. It is clear on reviewing the cited patent that the only cause of discomfort which has been considered is the lateral movement of the head during sleep, and that the origin has not been correctly addressed, neither has the weight of the head itself on the spinal column been taken into account as a source of discomfort when persons wish to sleep while seated. This is obviously a design which only makes it possible to rest partially and for brief periods, by counteracting lateral movements, since, by not including a support which neutralizes the direct weight of the head on the spinal column, and since the nape is not allowed to rest behind the line of support of the back, the musculature of the neck is not completely relieved, neither is the spinal column freed of the compressive weight of the head. Consequently, any comfort which is achieved by supporting the head laterally, will only be momentary and transient, since it will frequently be interrupted by the discomfort caused by the above-mentioned effects.
It also becomes obvious on reviewing the cited patent that the curved entry is prevented from holding the head upright, as is claimed; this is because it is of inadequate design, and because it does not have sufficient strength for such a role, due to the fact that it is produced from the same spongy and flexible material which forms the so-called "single plate". By the mere fact that the user is supporting his head towards the rear, the mouth of the semicircle will open out and automatically be deformed, ceasing to hold the neck in line. The proof of this is that the inventor has seen it necessary to produce a somewhat deep cavity provided with ventilation grooves, since the only way of at least lightly retaining the head of the user against the lateral movement is by introducing the whole of the head into the said cavity. Hence it is easy to prove that, since the "single plate" has no rigidity, if the user is sleeping and his head tends to fall sideways, the deformation of the whole of the "single plate" will mean that the whole of the load will fall on the half of the belt which fastens it to the seatback, while the remaining half will slacken off towards the direction of inclination of the head, and will cancel out the lateral containment effect for which the "single plate" was created.
In addition, it becomes clear on reviewing the cited patent that although the "single plate" was provided with elements for stiffening, that it has not been provided; it would also be missing from the appropriate design, at least so that the curved entry should serve as a point of neutralization of the weight of the head, since its straight and vertical walls would not offer any resistance in this direction, and the spinal column would still remain as the support of the heavy weight of the head.
However it is known that, in order to travel or rest during a journey, passengers use cushions for the purpose of achieving acomfortable position for the head, the neck and the spinal column, but that said cushions move, since they do not have a fixed point of support and do not define any anatomical shape, and they never allow the nape to be stretched out behind the back.